Active suspension assembly for a vehicle

[0001] This application is the U.S. National Phase of, and Applicants claim priority from, International Application Number PCT/EP2007/061366 filed 23 Oct. 2007 and European Patent Application No. 06076930.4 filed 23 Oct. 2006, all of which are incorporated herein by reference.

[0002] The invention relates to a suspension assembly for a vehicle, for suspending a second vehicle mass, such as a cabin or wheels, to a first vehicle mass, such as a chassis. More particularly, the invention relates to an active suspension assembly, arranged to actively counterbalance external loads acting on said suspended vehicle mass.

[0003] Such a suspension assembly is for instance known from WO 93/22150 and WO 2006/019298. The known assembly comprises a bearing arm, which at one end is pivotally connected to a chassis of the vehicle and at another end is associated to a cabin or wheel. Spring means are provided to produce a spring force that can exert a moment on the bearing arm, in order to counteract an external moment acting on said bearing arm due to for instance lateral, longitudinal and/or vertical accelerations of the vehicle (roll, pitch and heave respectively). Adjustment means are provided to alter a direction and/or the point of application of the spring force, so as to decrease or increase the counter moment on the bearing arm.

[0004] A disadvantage of this known suspension assembly is that the effective spring stiffness of the suspension assembly depends to a large extent on the instantaneous position of the point of application of the spring force. When this point of application is located near the pivot axis, the effective spring stiffness may generally be low, while it may be relatively high while the point of application is located near the opposite end of the bearing arm. Both situations may result in undesirable and/or uncomfortable drive behaviour.

[0005] It is therefore an objective of the invention to provide a suspension assembly of the above-described type, in which the disadvantage of the known assembly is overcome or at least reduced, while retaining the advantages thereof.

[0006] To this end a suspension according to the present invention is characterized by the features of claim 1.

[0007] Applicant has found, through simulations and testing, that the point of application of the spring force can be moved in a second direction, in which the counter moment will be not or hardly affected, but the effective spring stiffness will be affected. Accordingly, the effective spring stiffness can be controlled substantially independent of the counter moment. Undesirable changes in the effective spring stiffness, caused by moving the point of application of the spring force in a first direction to produce a desired counter moment, can be at least partly compensated, by moving the point of application in the appropriate second direction, over a predetermined distance. Thus, external load disturbances arising from lateral, longitudinal and/or vertical vehicle accelerations can be effectively suppressed, while at the same time the effective spring stiffness can be kept at a desired level.

[0008] According to an aspect of the invention, the adjustment means can be arranged to move the point of application of the spring force along a predetermined path, designed to maintain the effective spring stiffness at a substantially constant level, while varying the counter moment on the bearing arm. This may enhance driving comfort and furthermore may obviate the need for a complex, costly and energy consuming control mechanism that otherwise would have to be implemented to maintain the effective stiffness within desired boundaries.

[0009] According to an alternative embodiment of the invention, the adjustment means can be arranged to move the point of application of the spring force along a predetermined path, that causes the effective spring stiffness to increase, when the counter moment is increased in order to counteract an increased external load. Thanks to such increased effective spring stiffness, the natural frequency of the sprung mass can remain substantially constant, which again may contribute to the overall comfort of the vehicle. It furthermore allows the suspension assembly according to the invention to replace a conventional air spring, which features this same functionality.

[0010] According to an advantageous aspect of the invention the spring force may be exerted on the bearing arm via an elongated member, for instance a flexible string or a stiff rod. This allows the suspension assembly to be compactly built. An end of the elongated member may be received within the spring means, whereas the other end may be moved along a circular path, thereby causing the elongated member to describe a cone, or at least a part thereof.

[0011] Preferably, the elongated member is aligned with the centre line of the spring means under static load conditions. Thanks to such aligned position, radial or lateral loads on the elongated member are minimized. Especially frictional forces, acting between said elongated member and a bearing that supports the elongated member within the spring means will be minimized.

[0012] The invention furthermore relates to a vehicle equipped with a suspension assembly according to the invention, wherein the suspension assembly is used to suspend a cabin and/or the wheels of the vehicle to a chassis of the vehicle.

[0013] Further advantageous embodiments of an active vehicle suspension assembly according to the present invention and a vehicle equipped therewith are set forth in the dependent claims.

[0027] The basic inventive principle behind the present invention will first be explained with reference to a relatively simple, two dimensional suspension assembly 1, as shown in FIGS. 1A,B and 3. This suspension assembly 1 comprises a bearing arm 3 and spring means 5. The bearing arm 3 has a length L and is with one end pivotally connected to a chassis 4, around a pivot axis P. The other end is associated to for instance a wheel or a cabin 6 (as shown in FIG. 8). The spring means 5 exert a spring force F_s on the bearing arm 3 which can be moved along the bearing arm 3 (in X-direction) towards the pivot axis P or away there from, so as to decrease, respectively increase a moment M_c exerted on said bearing arm 3 by said spring force F_s. Thus, said moment M_c can be adjusted to counterbalance an external moment M_e that during use may act on the bearing arm 3. This displacement of the force F_s has effect on the effective stiffness C_eff of the suspension assembly 1. The effective stiffness C_eff is the stiffness that is felt between two points of the suspension assembly 1, in particular between point A where the spring means 5 are attached to the chassis 4 and point B where the bearing arm supports the cabin 6.

[0028] To show the effect of the displacement of force F_s on the effective stiffness C_eff of the suspension assembly 1, FIGS. 1A and 1C show two special, theoretical situations. In both situations, the spring means 5 exert a spring force F_s on the bearing arm 3, wherein spring force F_s is represented by following equation:
F_s=F₀+C_spring*u  (1)

[0029] with F₀ representing the pretension, C_spring representing the stiffness and u representing the elongation of the spring means 5.

[0030] In the theoretical situation according to FIG. 1A, the spring means 5 represents a ‘pure spring’, having a certain spring stiffness C_spring but no pretension (F₀=0). The effective stiffness C_eff at point B is given by the abovementioned equation (1), and can be seen to be independent of the position Δz of the pure spring. In the theoretical situation according to FIG. 1C, the spring means 5 comprise a ‘pure force’, having a certain pretension F₀ but no stiffness (C_spring=0).

[0031] It can be shown that the effective stiffness C_eff for both situations (FIG. 1A and C) can be expressed by following equation:

[0032] $\begin{array}{cc}{C}_{\mathrm{eff}}=\frac{C_{\mathrm{spring}}·\Delta x^2+F_0·\Delta z}{L^2}& \left(2\right)\end{array}$

[0033] From this equation it can be seen that a pure spring (FIG. 1A), with F₀=0 and C_spring≠0, only affects the effective stiffness C_eff when the spring is moved in X-direction, according to following parabolic relation:

[0034] $\begin{array}{cc}{C}_{\mathrm{eff}}=\frac{C_{\mathrm{spring}}}{L^2}\Delta x^2& \left(3\right)\end{array}$

[0035] This is illustrated in FIG. 1B, showing the effective stiffness C_eff to increase parabolic from zero to C_eff,max when the pure spring is displaced from the pivot axis P (Δx=0) to the end of the bearing arm 3 (Δx=L).

[0036] From equation (2) can furthermore be seen that a pure force (according to FIG. 1C) with F₀≠0 and C_spring=0 only affects the effective stiffness C_eff when this pure force is moved in Z-direction, according to following linear relation:

[0037] $\begin{array}{cc}{C}_{\mathrm{eff}}=\frac{F_0}{L^2}\Delta z& \left(4\right)\end{array}$

[0038] This is illustrated in FIG. 1D, showing the effective stiffness C_eff to increase linear from a negative value to a positive value, when the pure force is displaced from a position below the bearing arm 3 (Δz=Z_low<0) to a position above the bearing arm 3 (Δz=Z_high>0)

[0039] If a particular effective stiffness C_eff is desired as a function of the displacement in X-direction, it can be derived from equation (2) that the corresponding displacement in Z-direction should be equal to:

[0040] $\begin{array}{cc}\Delta z=\frac{1}{F_0}\left(C_{\mathrm{eff}}·L^2-C_{\mathrm{spring}}·\Delta x^2\right)& \left(5\right)\end{array}$

[0041] FIG. 2 represents the situation wherein the spring force F_s has been displaced in Z-direction over a distance Δz₁ and subsequently has been displaced along the bearing arm 3, from Δx=0 to Δx=L. This has been repeated for several other Z-displacements Δz_{2, . . . , 7}, wherein Δz₃ represents a Z-displacement of zero and Δz_4,5,6,7 represent negative Z-displacements, below bearing arm 3. From this FIG. 2 it can be seen that when the spring force F_s is displaced in X-direction (to increase a counteracting moment on the bearing arm 3) the effective stiffness C_eff of the suspension assembly 1 can be kept at a desired value C_eff,C by simultaneously displacing the spring force F_s downward in Z-direction over an appropriate distance Δz. The appropriate distance Δz is given by the intersecting points of the dashed line C_eff,C with the respective C_eff-curves. For instance, when the spring force F_s is displaced in X-direction from position Δx₂ to Δx₃, the spring force F_s should also be displaced in Z-direction from position Δz₂ to Δz₃. This is further illustrated in FIG. 3, showing the two-dimensional path along which the spring force F_s can be displaced, in X- and Z-direction so as to maintain a substantially constant effective stiffness C_eff.

[0042] It will be understood that in a similar way the effective stiffness C_eff can be controlled to vary linear with the X-displacement of the spring force F_s, or parabolic or according to some other desired curve. In such case the appropriate Δx,Δz-combinations can be found by plotting said desired curve in FIG. 2 and by noting down the coordinates of its intersection points with the C_eff-curves. Thus, the effective stiffness C_eff may for instance be controlled to increase proportionally with the external load so as to maintain the natural frequency of the suspended vehicle mass constant. This may facilitate various control facilities provided in the vehicle, for instance workspace control, and/or enhance driver comfort.

[0043] The invention is thus based on the insight that the effective stiffness C_eff and the counter moment M_c can be adjusted to a large extent independent of each other. Hence it is possible to displace a spring force F_s (or more particularly, its point of application P_a) towards or away from the pivot axis P of the bearing arm 3 to decrease, respectively increase the counter moment on said bearing arm 3, whereby unwanted variations in the effective suspension stiffness C_eff can be suppressed or reduced by simultaneously displacing said point of application P_a in an appropriate second direction.

[0044] This appropriate second direction will now be illustrated with reference to FIG. 9, showing a spring force F_s in an arbitrary position with regard to a bearing arm of which only the pivot axis P is shown. Pivot axis P extends along line L₁ and spring force F_s extends along line L₂. Lines L₁ and L₂ cross, but do not intersect. Between lines L₁, L₂ a shortest distance W can be defined, extending along line L₃, perpendicular to lines L₁ and L₂. Furthermore, two orthogonal planes V₁ and V₂ can be defined, wherein V₁ includes spring force F_s and extends parallel to line L₁ and V₂ includes line L₃ and extends perpendicular to line L₁. The planes V₁, V₂ intersect along line L₄, which line L₄ includes an angle α with spring force F_s. The latter can be resolved in a component F_s*cos α, extending parallel to line L₄ and a component F_s*sin α extending perpendicular thereto (not shown). The first component exerts a counter moment M_c on the bearing arm, according to following equation:
M_c=W*F_s*cos α  (6)

[0045] As this equation includes no reference to the position of the point of application P_a of the spring force F_s, said position can be freely shifted within plane V₁, without affecting the counter moment M_c, provided that the angle α between F_s and line L₄ is kept constant. By shifting said point of application P_a along plane V₁, the distance h between line L₃ and the projection of the point of application P_a on plane V₂ will change (except when P_a is shifted parallel to line L₁). By changing said distance h, the effective stiffness of C_eff will be affected, according to following equation (7) (which resembles equation (2), derived for the two-dimensional (2D) embodiments of FIGS. 1A,C):

[0046] $\begin{array}{cc}{C}_{\mathrm{eff}}=\frac{\left(C_{\mathrm{spring}}·{\cos }^2\alpha \right)\Delta x^2+\left(F_0\cos \alpha \right)·\Delta z}{L^2}& \left(7\right)\end{array}$

[0047] For FIG. 9, Δx equals W and Δz equals h. Thus, in the embodiment according to FIG. 9, ‘displacement of the point of application P_a in an appropriate second direction’ includes any displacement in plane V₁ provided said displacement is not parallel to L₁ and provided the angle α between force F_s and line L₄ is kept constant.

[0048] Thus, for every point of operation, a unique combination of parameters W and h, or more generally Δx and Δz, can be determined, which combination at the one hand produces a desired counter moment M_c according to equation (6) and at the other hand produces a desired effective stiffness C_eff according to equation (7).

[0049] Of course, in a similar way, it is also possible to alter the effective stiffness C_eff without substantially affecting the counter moment M_c on the bearing arm 3.

[0050] The above discussed inventive insight is applied in the schematic suspension assembly 101 according to FIGS. 4A-D. Parts similar to those in FIG. 1 have been denoted with similar reference numerals, increased by 100. The suspension assembly 101 comprises a triangular bearing arm 103, which is pivotally connected to a chassis 104, around a pivot axis P, and spring means 105, which are likewise attached to the chassis 104, at some distance above the bearing arm 103. The spring means 105 exert a spring force F_s on the bearing arm 103 via an elongated member 108, such as for instance a rod or a string. Furthermore, adjustment means are provided (not shown), which can move an end of said elongated member 108 along a circular path C, thereby having the elongated member 108 describe a cone with centre line M. The member end can be rotated in clockwise direction (indicated by arrow −γ) or counter clockwise direction (indicated by arrow +γ) from a neutral position (indicated by γ₀), in which the elongated member 108 engages the bearing arm 103 at a certain distance from the pivot axis P, typically corresponding to a position in which the spring force F_s, in particular the pretension part F₀ thereof (cf. equation (1)) is in equilibrium with the static load acting on the free end of the bearing arm 103 (due to gravity).The rotation in +γ and −γ direction will cause the spring force F_s to travel away from and towards the pivot axis P, thereby not altering the spring force Fs as such, and the counter moment M_c exerted on the arm 103 to decrease, respectively increase. As explained in relation to FIGS. 1-3, such displacement of the spring force F_s may cause the effective stiffness C_eff of the suspension 101 to vary.

[0051] In the given example, this is avoided by lifting the point of application P_a of the elongated member 108 over a distance Δz above the bearing arm 103, and by tilting the aforementioned centre line M of the cone slightly forward, around an axis extending parallel to the X-axis, and sideward, around an axis extending parallel to the Y-axis, so as to respectively include an acute angle Θ and an acute angle χ with the normal of the bearing arm 103 (as best seen in FIGS. 4D and 4A respectively). Thanks to this tilted orientation, the point of application P_a of the elongated member 108 will move up en down in Z-direction when it is travelled along the circular path C, in accordance with the aforementioned inventive insight. Consequently, the effective stiffness C_eff of the suspension assembly 101 can be kept substantially constant, at least over a considerable range of the bearing arm 103, as shown in FIG. 5, wherein the effective stiffness C_eff only rises when the point of application P_a of the elongated member 108 approximates the pivot axis P and the opposite end of the bearing arm 103. For comparison, the dashed line represents the effective stiffness C_eff in case where the centre line M is not tilted but extends perpendicular to the triangular plane of the bearing arm 103.

[0052] Of course, the amount of displacement in the Z-direction for any given position of the spring force F_s can be influenced by changing the or each tilt angle Θ, Φ and/or the tilt axes of the centre line M. Thus, the graph of the effective stiffness C_eff, as shown in FIG. 5, can be changed as desired, in a similar way as explained for the suspension assembly 1 of FIGS. 1-3.

[0053] It is furthermore observed that in the present embodiment, for rotations of the elongated member 108 in the −γ direction the displacement ΔZ is positive, that is upward from the bearing arm 103, and for rotations in the +γ direction ΔZ is negative, that is downward from the bearing arm 103.

[0054] FIGS. 6A and B show a possible embodiment of the schematic suspension 101 of FIG. 4. Like parts have been denoted with like reference numerals. The embodiment is shown to comprise a bearing arm 103, which is pivotally connected to a chassis 104 around a pivot axis P. A coupling rod 106 is provided near its opposite end, for connection to a vehicle mass, for instance a cabin (see FIG. 8). Furthermore, spring means 105 are provided, mounted some distance above the bearing arm 103, on a support bracket 107 of the chassis 104, wherein a centre line S of the spring means 105 extends substantially perpendicular to the bearing arm 103 (under static load conditions, with the arm 103 extending substantially horizontal). The spring 105 exerts a spring force F_s on the bearing arm 103 via an elongated member 108, which in the given example is designed as a pull rod but in alternative embodiments could be a flexible chord or string. The rod 108 has a bend shape, with a first end extending centrally through the spring 105 and the support bracket 107, and a second end extending from said support bracket 107 radially outward to the bearing arm 103. A cover plate 109 is seated on top of the spring 105 and connected to the pull rod 108 so as to compress the spring 105 against the support bracket 107, thereby causing pull rod 108 to be preloaded with a pulling force F_s. Of course, the connection between the cover plate 109 and the rod 108 may be adjustable, so as to allow said force Fs to be adjusted.

[0055] The pull rod 108, at its lower end is connected to adjustment means 110, more particular a rotary motor, designed to rotate said rod end along a segment of a circular path C, with centre line M. According to the invention, the motor 110 is fixated to the bearing arm 103 so as to have the centre line M tilt slightly forward relative to the plane of the bearing arm 103, i.e. around the X-axis. The centre line M may furthermore be tilted slightly sideward, to the left, around the Y-axis. Consequently, when travelling said circular path C, the end of the rod 108 will displace in X-direction (to and away from the pivot axis P) as well as in Z-direction. As explained in relation to FIG. 4, such displacement pattern may help to minimize variations in the effective stiffness C_eff of the suspension.

[0056] To guide the rod movement in Z-direction, the upper end of the pull rod 108 is connected to a second arm 113, which can pivot in parallel to the bearing arm 103 and as such can function as a parallel guide mechanism. In order to allow the upper end of the pull rod 108 to rotate around the centre line S, relative to the second arm 113 and the spring means 105, use can be made of spherical bearing parts.

[0057] Preferably, the suspension 101 is dimensioned such that under static load conditions, i.e. when the vehicle is standing still or rides stationary and no acceleration or deceleration forces act on the suspended vehicle mass, the bearing arm 103 extends horizontally and the pull rod 108 extends halfway the bearing arm 103, in a neutral γ₀ position as shown in FIGS. 4 and 6A. In this position, the moment M_c exerted on the bearing arm 103 by the spring force F_s counterbalances the moment M_e exerted on the bearing arm 103 by the load F_e of the suspended vehicle mass 106. Furthermore, the tilt angles Θ and Φ of centre line M (see FIG. 4) are preferably chosen such that under said static load conditions the elongated member 108 extends substantially in line with the centre line S of the spring means 105 and substantially perpendicular to the bearing arm 103. This may help to minimize the radial forces acting on the elongated member 108 and reduce wear and friction between the member 108 and the guiding means that support the member 108 within the spring means 105.

[0058] The assembly 101 may further comprise a sensor (not shown) to measure one or more parameters that are representative for the external load ΔF_e, ΔM_e on the bearing arm 103, such as for example an acceleration, velocity and/or displacement of the first vehicle mass 106. These measurement data are provided to a central control unit (not shown) that, based on these data, a desired load profile and/or a preprogrammed control strategy controls the adjustment means 110 to rotate the pull rod 108 clockwise or counter clockwise so as to decrease, respectively increase a distance Δx between the spring force F_s and the pivot axis P, thereby allowing the dynamic load to be balanced, partly or completely, depending on the control strategy. During use, when driving over road irregularities, the cabin 106 must be able to move up and down with respect to the chassis 104. In this manner, road induced vibrations are absorbed by the suspension assembly 101. These relative motions cause the bearing arm 103 to rotate around the pivot axis. Consequently, the pull rod 108 will move upward or downward (depending on the direction of rotation), thereby guided by the second arm 113. This will cause the distance between the cover plate 109 and the support bracket 107 to increase or decrease, the spring 105 to relax or tension and consequently the spring force F_s to increase or decrease.

[0059] FIG. 7 shows an alternative embodiment of a suspension 101 according to the invention. This embodiment differs from the embodiment shown in FIGS. 6A,B mainly in the way that the Z-movement of the elongated member 108 is facilitated. To that end, the support bracket 107 that supports the spring means 105 is designed to have a bushing 115 extending within said spring means 105. The elongated member 108 is made of an upper rod 108A and a lower rod 108B, which are linked together via a universal joint 114, somewhat halfway the bushing 115. Said joint 114 allows the upper rod 108A to extend centrally within the bushing 115 and the surrounding spring means 105, in alignment with the centre line S, whereas the lower rod 108B extends radially downward and outward, from the bushing 115 toward the bearing arm 103. The upper rod 108A is near its upper end connected to a cover plate 109, so as to compress spring 105 against said support bracket 107, and the lower rod 108B is near its lower end connected to adjustment means 110, in a similar fashion as described for the embodiment of FIGS. 6A,B. The universal joint 114 is encased within a cylindrical block 116 that is dimensioned to slide within the bushing 115 and thus can act as a bearing for guiding the rod 108 during its displacement in Z-direction. The working of the suspension 101 is similar to the embodiment of FIGS. 6A,B.

[0060] The invention is not in any way limited to the exemplary embodiments shown in the description and the figures.

[0061] For instance, the inventive insight may likewise be applied to ‘non supporting’ suspension assemblies, which in neutral position exert no counter moment M_c on the bearing arm, like the embodiments disclosed in WO 93/22150. To support the external load in said neutral position, these type of suspension assemblies are equipped with an additional spring leg.

[0062] In the embodiment according to FIG. 1 of WO/93/22150, the inventive thought can be incorporated by tilting one of the guides (in which the spring 18 is moved sideward, parallel to itself) around an axis extending substantially perpendicular to the plane of the paper. In the embodiment according to FIG. 4 of WO 93/22150, the cone may be offset in the same directions and tilted around the same angles as discussed with regard to FIG. 4 of the present description. And in the embodiment according to FIG. 5 of WO 93/22150, the cone described by the auxiliary spring means 18, may be tilted in accordance with the present invention, by rotating the fixed hinge point 17 around the centre point of the circular base of the cone, in a plane that is defined by the centre line 33 of the cone and the centre line 11 of the two armed lever 20.

[0063] It is furthermore noted that in all embodiments described, the spring means could be rearranged to exert a pressure force on the bearing arm, instead of a tensile force. In such case, the point of application P_a of the spring force F_s should be moved in the opposite direction as to what has been described above, i.e. to increase the effective spring stiffness Δz should be negative.

[0064] All combinations (of parts) of the embodiments shown and described are explicitly understood to be incorporated within this description and to fall within the scope of the invention. Moreover, many variations are possible within the scope of the invention, as outlined by the following claims.